The time between the 1600s and the Civil War was a very important time in US history.  The US grew a ton through the Northwest Ordinance in 1787, the Louisiana Purchase in 1801, the Missouri Compromise in 1820, the Treaty of Guadalupe Hidalgo in 1829-1848, and the Gadsden Purchase in 1853.  The US also faced a lot of challenges. The first government the US had was called the Articles of Confederation this did not work out because the government had no power to tax the states and every vote had to be unanimous or it did not pass.  The states remained unified up until the Civil War because they relied on each other to fend off the British.
                                                                                                                                               Wyatt